PLSO The Oregon Surveyor May/June 2021

Featured Article 15 Professional Land Surveyors of Oregon | www.plso.org Preparation for the Journey Lewis and Clark and their Corps of Discov- ery have been referred to as “thewritingest” group of explorers to undertake a major overlandexpedition. The same thing canbe said for their mapmaking. They prepared about 140 maps on the trail and collected some 30 maps from Indians, fur trappers, and traders. Like writing, mapmaking re- quires special aptitude and training. But while nine of the 33 participants kept jour- nals, only one drew maps. Most of themaps were drawn or compiled by William Clark. Although Clark had little formal education, he displayed an inher- ent ability for mapmaking. He appears to have learned surveying and mapping on the Virginia frontier and in the Army. His first teacher was probably his older brother, the famous Indian fighter and Revolutionary War hero George Rogers Clark. His brother was also a land surveyor. As a young Army officer during the 1790s, Clark received further training inmapmak- ing. He was responsible for constructing military fortifications, a job that required skill in drawing and reading maps. While Meriwether Lewis was not a car- tographer, he carried out much of the celestial observation. He was tutored in this activity by some of America’s leading scientists, mathematicians, and surveyors. These included the third President of the United States and members of the Amer- ican Philosophical Society. Using his own instruments, Thomas Jefferson personally taught Lewis the basic principles of deter- mining latitude by observing altitudes of the sun or a star with an octant. Jefferson also sent Lewis away for further study. He learned about calculating astro- nomical formulas from Robert Patterson, professor of mathematics at the Universi- ty of Pennsylvania. From Andrew Ellicott he learned field surveying. Ellicott was the foremost surveyor of his day. He had conducted numerous frontier boundary surveys, including the boundary between the United States and the Spanish posses- sions in Florida. In 1791, Ellicott prepared the first printedmap of the nation’s capital. No other American had more experience inmaking celestial observations in the field under trying circumstances. Ellicott pre- pared a list of the instruments that would be required to properly survey and map the explorers’ route, and he instructed Lewis in their use. The two explorers used their first winter together at Camp Dubois (Wood) and in St. Louis improving their surveying skills and collecting geographical information about the route they planned to follow. FromDec. 12, 1803, to May 14, 1804, Clark, who was in charge of Camp Dubois, practiced ce- lestial observation andmapmaking. At the same time, Lewis conferred with Spanish and French surveyors in St. Louis, gather- ing information and copying their maps. During the expedition, Clark prepared four types of trail maps. These were large-scale compass traverse maps, small page-size maps sketched in his journals, copies of maps prepared by traders and Indians, and composite maps of the West. Compass Traverse Maps The primary maps prepared by Lewis and Clark were called compass traverse maps. These show the route that they traveled each day. The first step in preparing these maps required the determination of geo- graphical latitude and longitude by celestial observation. This was done at selected places such as where major rivers joined the Missouri or Columbia rivers. A variety of instruments were used for this purpose. Longitude was normally determined with a chronometer (clock). To maintain ac- curate time, the chronometer had to be wound each day at noon. Sometimes the explorers forgot or did not have time to wind it. To reset the chronometer, the explorers had to take additional celestial observations. Lewis also tried to determine longitude by celestial observation, but without much success. This was a tedious job that re- quired as many as three assistants and took most of an evening. The explorers weremore successful deter- mining latitude, which involves measuring the angle of the moon and a star with an instrument called an octant. Latitude was usually determined for each camp site. Other instruments that the explor- ers used in celestial observation were the sextant and artificial horizon. Next, Lewis and Clark recorded the direc- tion and distance covered for each leg of their journey. Direction was determined with a “circumferentor” or plain surveyors compass. Distances from point to point were generally estimated in miles. Clark andmany of the soldierswere experienced woodsmen who were trained from child- hood to estimate distances. In some instances, however, the Captains wantedmore accuratemeasurements. For this they used a surveyor’s chain made of iron links, each measuring 7.92 inches in length. The entire chain measured two poles (33 feet) and was called a “two pole Chain.” The “pole” was a common unit of measurement during the early nineteenth century. One pole equaled 16 1/2 feet. The explorers also carried a “log line reel” for measuring speed and distance trav- eled on water. Although no description survives, log lines generally consisted of a rope line divided into known lengths. This line would be attached to a buoy or other stationary object in thewater near the boat. As the boat moved forward, the explor- ers could then time the interval required for the line to be played out. The log line would also be used to determine the rate of the current. Why was this important? The distance and direction for each seg- ment of the trip were recorded daily in their journals and field maps in the form of a table which Clark labeled “course and distance.” A portion of a course and distance table is shown below. It covers a segment of their route for June 15, 1805. On this day they traveled twelve miles. The first column expresses the compass bearing (course or direction) for the dis- tance indicated. Bearings are expressed in degrees fromdue north or south. Look at the second line that begins with “S 28º W” or South 28 degrees West . This means that the expedition traveled along a route 28 degrees west of due south for two and one-fourth miles to a point in the river marked by a large rock in the shape of a tower. This rock was located on the right side of the river. South 1 1/2 (miles) to a point on the Stard [starboard or right] Side S28ºW 2 1/4 (miles) to a rock resembling a tour [tow- er] in the Stard [Starboard] bend S 10º E 1 3/4 (miles) to the Stard. point passing a rapid S60ºW 3/4 (miles) to a tree in the Std [Starboard] bend rocks & rapds [rapids] all the dist[ance] South 3/4 (miles) to some bushes on a Lard [lar- board or left side] point passed a large Creek at 1/2 a mile on the Lard. Side which we called Shield’s Creek [named after John Shields, whom Lewis and Clark sent to ex- plore this river] continues T